There's no fixed answer to that. The best bike would be the one best suited to your riding, your size and your wallet. If you want one that doesn't suck horribly if you have to ride it a few miles on...
It's no trouble at all using supposedly rim brake rims with disc brakes, nothing will happen because of that. But your hubs has to be able to take a brake rotor and your fork/frame has to be able to...
That really depends on what you want the bike for. For downhill / all-mountain, a full suspension bike is preferable. For cross country a hardtail can still be competitive, particularly for shorter...
Kinda-sorta. If you're willing to pay for it you can have a frame builder add disc brake mounting tabs on any bicycle frame and fork.If you're not willing to pay for it you'll have to settle for...